Land Use and Geo-Engineering - Introduction to Carbon Capture and Sequestration - page 531

Box 11.3.1 Biochar
Biochar is obtained by pyrolysis of biomass. Pyrolysis is heating in a low- (or no-)
oxygen environment. The lack of oxygen prevents the combustion of the biomass.
Pyrolysis is the process that occurs when we roast food, giving it the brown color. If
we roast at too high a temperature, the food becomes black and we have produced
a form of biochar. In fact, pyrolysis has been used for centuries to produce charcoal.
To convert biomass, we can use different forms of pyrolysis. Depending on
the temperature and the length of the pyrolysis process we can produce a mixture
of bio-oils, biochar, and syngas. High temperature pyrolysis is also known as gasi-
fi cation. Temperatures of 400-500°C produce more char, while temperatures
above 700°C favor the yield of liquid and gas fuel components. The fi gures show
some examples of biochar.
Left: This char has a carbon content of 87% and has been shown to detoxify the
more serious afl otoxins plaguing US agricultural lands.
Right: Biochar made from rotting palm trees, from weeds which are otherwise
burned in the open air, and from cassava stems, are gathered and burned by
farmers to avoid uncontrolled growing in the farm and soil nutrient depletion.
Images courtesy of Christophe Steiner: http://www.biochar.org
From a technological point of view, no fundamental diffi culties pre-
vent the operation of a large scale biochar program in which we perma-
nently sequester the carbon that would otherwise be emitted to the
atmosphere. At present, however, our knowledge of the potential nega-
tive effects is incomplete and much more research is needed.
Enhanced weathering
In nature's carbon cycle (Chapter 3), the temperature of the earth is regu-
lated by CO 2 levels. If the temperature is high the weathering of rocks